I, also, will concur with most of what I have read here. Just because the behavior appears to be manipulative and attention-seeking to you, it does not mean that your sister is not in a great deal of emotional pain and in need of support.

I do, however, want to add something supportive for you. As you describe them, her behaviors are not appropriate ways to express her emotional pain or obtain the reassurance and attention that she desires. Proper diagnosis and treatment could help her to learn more appropriate ways of coping. If she wishes to get help you should support her as much as you are capable of doing so. If she does not wish help, it may be painful but necessary to try to not react to her behaviors. I am not encouraging you to ignore the pain she obviously feels, but to not enable and encourage behaviors that are destructive to your relationship with her. Ask her about what she really feels, try to talk through problems with her, but try not to react emotional to the inappropriate behaviors (provided they are not life threatening). You can encourage someone to seek help, but you cannot make her do it until she is ready and willing.

One more note, if she has an alcohol or drug problem perhaps she would be more open to something like AA than to professional therapy. It may not be all that she needs, but any place to start is better than nothing. If it opens the door to seeing that a problem exists it may allow her to see that she also needs professional help!